(1) Field of the Invention
The present invention relates to a transmission method and apparatus which transmits low-speed SDH (synchronous digital hierarchy) signals using a high-speed SDH frame.
(2) Description of the Related Art
In order to exploit the immense bandwidth available for transmission in an optical fiber, research in wavelength-division multiplexing (WDM) technology is driven. However, the bandwidth utilized by the WDM cannot be directly utilized by time-division multiplexed (TDM) systems due to the speed limitations posed by fiber dispersion. The WDM offers a practical solution of multiplexing many high-speed channels at different optical carrier frequencies and transmitting them over the same fiber. WDM systems are beginning to be deployed more widely in the field and are expected to play a greater role in the near future.
There has been rapid advances in the Internet technology in recent years. The demand for increasing the per-fiber transmission capacity of telecommunications networks continues to grow and the need for economical capacity upgrades becomes compelling. The existing TDM systems are not suitable to meet the higher-capacity demand due to the speed limitations. The WDM systems offer a practical solution of increasing the network transmission capacity and are suited for supporting the high-capacity demand. Recent developments of add-drop multiplexers and digital cross-connect systems have made possible a tremendous increase in the per-fiber transmission capacity.
However, there are at least two problems which relate to the WDM technology. The first is that the dispersion shifted fiber (DSF), widely deployed in the field, is generally suitable for long-haul transmissions of high-speed signals but is not suited for the WDM systems. For this reason, when deploying the DSF, a high-speed TDM system is needed. The second problem of the WDM technology is that the WDM technology offers a bit-rate-free transmission but the efficiency of transmission attained by the WDM is higher when many high-speed signals (e.g., 10 Gbps) are transmitted than when low-speed signals (e.g., 2.4 Gbps) in the same number are transmitted. In order to meet the high-capacity demand, the higher transmission efficiency is preferred.
By taking account of the above-mentioned matters, currently available upgrade options are: use of higher speed electrically-multiplexed systems; use of additional fibers; and use of wavelength multiplexing. Although a mix of three approaches will most likely be used in different parts of the network depending on the need and the economics, a transmission technology which utilizes high-speed electrically-multiplexed systems that operate in a transparent manner similar to the WDM systems is especially attractive. Since cost saving is made possible through fiber and amplifier sharing, the demand for such a technology becomes more compelling.
FIG. 13 shows a configuration of high-speed WDM systems which are connected to low-speed electrically-multiplexed systems via working and protection optical-fiber cables.
In the configuration of FIG. 13, reference numerals 1(1) through 1(4) denote 2.4-Gbps electrically-multiplexed systems which includes four working channels #1 through #4 and one protection channel, both having a data rate of 2.4 Gbps. Reference numerals 2(1) through 2(5) denote high-speed WDM systems which use the wavelength multiplexing. Various optical-fiber cables accommodating the working and protection channels are connected between the 2.4-Gbps systems and the WDM systems. For example, an optical signal sent from one (the working channel #1) of the working channels #1-#4 of the 2.4-Gbps system 1(1) reaches the WDM system 2(1), and the WDM system 2(1) outputs a wavelength-multiplexed optical signal. Optical signals sent from the protection channels of the 2.4-Gbps systems 1(1) and 1(4) reach the WDM system 2(5), and the WDM system 2(5) outputs a wavelength-multiplexed signal.
The WDM configuration of FIG. 13 offers a bit-rate-free transmission and makes the signal processing simple. When a failure of any of the WDM systems 2(1) through 2(4) occurs, the faulty WDM system is substituted for by the WDM system 2(5) by suitably switching the optical-fiber cables.
In the WDM systems 2(1) through 2(5) of FIG. 13, the failure recover capability of 1 protection channel to 4 working channels is provided since all the optical signals of the working channels #1-#4 and the protection channel are transported to these WDM systems.
FIG. 14 shows a configuration of a high-speed electrically-multiplexed system which is connected to a low-speed electrically-multiplexed system.
In the configuration of FIG. 14, reference numerals 1 denote 2.4-Gbps electrically-multiplexed systems which include four working channels #1 through #4 and one protection channel, both having a data rate of 2.4 Gbps. The two low-speed systems 1 are connected by an optical-fiber cable containing the working channels #1 through #4 and the protection channel. Reference numeral 3 denotes a 10-Gbps electrically-multiplexed system which has a data rate of 10 Gbps. In the high-speed system 3, the four 2.4-Gbps signals sent by the low-speed system 1 are electrically multiplexed into a 10-Gbps high-speed signal, and then the high-speed multiplexed signal is converted into an optical signal. The high-speed system 3 transmits the optical signal on a first optical-fiber cable as the working channel, and regenerates an optical signal on a second optical-fiber cable as the protection channel.
In the high-speed electrically-multiplexed system 3 of FIG. 14, only the failure recover capability of 1 protection channel to 1 working channel can be provided since the low-speed signals of the working channels #1-#4 sent by the low-speed system 1 are terminated at the input of the system 3.
In the configuration of FIG. 14, the low-speed system 1 which includes the four working channels #1 through #4 and one protection channel has a failure recovery capability of 80%, but the high-speed system 3 has only the failure recovery capability of 50%. That is, there is a problem in that the use of the high-speed electrically-multiplexed system 3 will lower the failure recovery capability. Further, when the vender of the low-speed systems 1 is different from the vendor of the high-speed system 3, the configuration of FIG. 14 does not necessarily assure the compatibility between the low-speed systems 1 and the high-speed system 3.
By taking account of the above-mentioned matters, the demand for a transmission technology which utilizes high-speed electrically-multiplexed systems that operate in a transparent manner similar to the WDM systems becomes more compelling than before.
The fundamental concept to meet the above-mentioned demand for the WDM-like transmission technology is that four low-speed signals are simply multiplexed into a high-speed signal at a transmit-side network element and the high-speed signal is demultiplexed at a receive-side network element. For example, FIG. 12 shows a multiplexing of four 2.4-Gbps signals into a 10-Gbps signal and demultiplexing of the 10-Gbps signal.
As shown in FIG. 12, a parallel-to-serial conversion (P/S) of 4 inputs to 1 output is provided at a transmit-side network element to multiplex the 2.4-Gbps signals #1 through #4 of the four channels into a 10-Gbps signal. This 10-Gbps signal is serially transported on an optical-fiber cable to a receive-side network element. A serial-to-parallel conversion (S/P) of 1 input to 4 outputs is provided at the receive-side network element to demultiplex the 10-Gbps signal into the reconstructed 2.4-Gbps signals #1 through #4.
The correspondence between the reconstructed low-speed signals and their channels is unknown to the receive side of the network, and it is necessary that the number of channel for each of the input 2.4-Gbps signals is carried on the overhead of the 10-Gbps signal before the transmission. On the receive side of the network, the frame synchronization is performed for a single channel of the low-speed signals reconstructed at the output of the S/P conversion with respect to a corresponding one of the known channel numbers. The channel number of the reconstructed low-speed signal of each channel is detected in this manner, and the channel allocation is controlled based on the detected channel number. The concept of the multiplexing method of FIG. 12 is analogous to the initial concept of the multiplexing method of the SONET. See TA-TSY-00253, Issue 2 published by Bellcore.
The above-mentioned demand for the WDM-like transmission technology can be met by using the P/S and the S/P conversion as shown in FIG. 12. However, there are at least two problems with the multiplexing method of FIG. 12.
The first problem is that the simple multiplexing of four 2.4-Gbps signals into a 10-Gbps signal does not assure the high-speed line error monitoring. In a case of the WDM systems, the high-speed line error monitoring must be performed by using an optical power monitoring device or an optical spectrum-analyzer.
In a case of the electrically-multiplexed systems, the high-speed line error monitoring must be performed at the 10-Gbps systems by detecting the B1 byte (bit interleaved parity code) in the section overhead of a STS-N frame or a STM-N frame. Further, accessing the SDCC (section data communications channels) bytes in the section overhead of the high-speed SDH frame by the 10-Gbps systems is required to make it possible to remote control the optical transmission network elements by a control unit such as a workstation. For these reasons, it is difficult for the conventional systems to assure the high-speed line error monitoring when transmitting the low-speed SDH signals by using a high-speed SDH frame.
The second problem of the multiplexing method of FIG. 12, is that the simple multiplexing of four 2.4-Gbps signals into a 10-Gbps signal does not directly assure the clock error adjusting. When a significant clock error between the incoming clock and the outgoing clock exists at the 10-Gbps systems, the clock error adjusting must be performed before transmitting the high-speed SDH frame.
For example, FIG. 15 shows an optical transmission ring network in which high-speed transmission systems and a low-speed transmission system coexist. In this ring network, the 10-Gbps signal is transported on a main optical-fiber cable between the 10-Gbps systems. If the demand for transmitting 2.4-Gbps low-speed signals in a portion of the ring network by using the 10-Gbps high-speed signal occurs, the exact synchronization between the clock of the 10-Gbps systems and the clock of the 2.4-Gbps systems is not necessarily assured by simply placing the existing 2.4-Gbps low-speed systems in the network portion.
The above demand may frequently occur when it is desired to upgrade the existing 2.4-Gbps systems of a vendor to the 10-Gbps systems of a different vendor simultaneously with the upgrading from the existing 2.4-Gbps optical-fiber cables to 10-Gbps optical-fiber cables.
In the SONET (synchronous optical network), a clock frequency error of xc2x120 ppm between the incoming clock and the outgoing clock at a network element is allowed, and the AU (administration unit) pointers (the H1, H2 and H3 bytes) of the SONET frame are used to compensate for the possible frequency difference between the incoming clock and the outgoing clock. However, the AU pointers are located in the line overhead of the SONET frame not in the section overhead thereof, and the use of the AU pointers does not directly assure the high-speed line error monitoring nor the clock error adjusting.
Accordingly, it is difficult for the conventional systems to assure the clock error adjusting and the high-speed line error monitoring when transmitting the low-speed SDH signals by using a high-speed SDH frame.
An object of the present invention is to provide an improved SDH signal transmission method and apparatus in which the above-mentioned problems are eliminated.
Another object of the present invention is to provide a transmission method which executes the transmission of low-speed SDH signals using a high-speed SDH frame and ensures the clock error adjusting and the high-speed line error monitoring.
Another object of the present invention is to provide a transmission apparatus which executes the transmission of low-speed SDH signals using a high-speed SDH frame and ensures the clock error adjusting and the high-speed line error monitoring.
The above-mentioned objects of the present invention are achieved by a transmission method which transmits low-speed SDH signals using a high-speed SDH frame, the transmission method including the steps of: multiplexing the low-speed SDH signals into the high-speed SDH frame, the high-speed SDH frame including an information payload, a line overhead and a section overhead, the section overhead being divided into a first section overhead SOH and a second section overhead SOH, the first SOH carrying regenerator SOH bytes and the second SOH carrying multiplex SOH bytes; detecting the multiplex SOH bytes in the second SOH of the high-speed SDH frame without changing the line overhead and the payload when the high-speed SDH frame reaches a receive-side high-level line terminating equipment; and generating the multiplex SOH bytes in the second SOH of the high-speed SDH frame without changing the line overhead and the payload before the high-speed SDH frame is transmitted by a transmit-side high-level line terminating equipment.
The above-mentioned objects of the present invention are achieved by a transmission apparatus which transmits low-speed SDH signals using a high-speed SDH frame, the transmission apparatus including: a multiplexer which multiplexes the low-speed SDH signals into the high-speed SDH frame, the high-speed SDH frame including an information payload, a line overhead and a section overhead, the section overhead being divided into a first section overhead SOH and a second section overhead SOH, the first SOH carrying regenerator SOH bytes and the second SOH carrying multiplex SOH bytes; a multiplex SOH detecting unit which detects the multiplex SOH bytes in the second SOH of the high-speed SDH frame without changing the line overhead and the payload when the high-speed SDH frame reaches the transmission apparatus; and a multiplex SOH generating unit which generates the multiplex SOH bytes in the second SOH of the high-speed SDH frame without changing the line overhead and the payload before the high-speed SDH frame is transmitted by the transmission apparatus.
In the transmission method and apparatus according to the present invention, the multiplex SOH bytes in the high-speed SDH frame are created and used by the high-speed line terminating equipment for clock error adjusting, high-speed line error monitoring, and high-speed line level equipment communications. The transmission method and apparatus of the present invention is effective in assuring the clock error adjusting and the high-speed line error monitoring when transmitting the low-speed SDH signals by using the high-speed SDH frame.